Conventional tree shears employ one or two blades. In two blade shears both can be movable toward one another or one can be fixed and the other move against it. One blade may also move against an anvil. The blade or blades are actuated by one or more hydraulic cylinders. Oil flow for the cylinders is normally provided by a fixed displacement hydraulic pump powered by an internal combustion engine-all as is well known in the art.
Since the mid 1970's, very little work has been done to improve the design of tree severing shears. This has been largely due to the demand for saw severing devices. There has been, however, some rebound in the demand for shears because of the high cost, and hazardous nature of saw operation. The present invention embodies part of my response to this demand.
In 1976, the Canadian Forestry Service published Report FMR-X-93 "Analysis of Shear Felling Trees". It summarized the history and state-of-the-art in tree severing shears and proposed some concepts which the author felt would improve their efficiency by more closely matching the shear force available to that actually required. In my view only one concept showed promise of a significant improvement in the area of force matching. It employed three cylinders, variable length links and a control system for automatically adjusting link lengths. This design has the potential for exact force matching but because of its complexity and bulkiness it is of doubtful utility.
A force matching shear is a shear in which the force available matches the force required as the blade or blades pass through the tree bole. This is illustrated in FIG. 24 of the FMR-X-93 Report. The minimum available force must be at least that force required to sever the largest tree bole for which the shear is designed. Since all components of the shear must be capable of transmitting this force the optimum in force matching is to match the minimum available force.
Virtually all known shears use a cylinder to push the blade or blades through the tree. No force penalty is incurred, because of the rod, as would be the case if the cylinder pulled the blades. In 1978 I considered a design which accepted this penalty in exchange for completely enclosing the cylinder in the housing. The configuration was acceptable at low force requirements but as these requirements increased cylinder dimensions reached impractical proportions and the design was abandoned.
The Felling Head disclosed in my Canadian Patent No. 1,103,130 issued Jun. 16, 1981 has been in commercial use for some years. Commercial pressures are forcing a re-examination of this design and I have reviewed the 1978 work discussed above. This review has led to incorporation of a novel cylinder and lever system which is simple, compact, and which, for all practical purposes, provides optimum force matching.